The present invention relates generally to adaptive spectral, spatial and temporal sensing in imaging applications and is especially useful in two-dimensional imaging arrays.
Conventional cameras and photographic systems allow limited-colors in images. Color cameras typically collect information in broad spectral bands assigned to each color, and these spectral bands are fixed. There are also imaging systems which operate in spectral bands outside of the visible range, such as in the infrared spectral region. Such imaging systems are very useful in that they enable thermal imaging using the emitted infrared radiation from warm bodies, for example, and specific spectral bands in the infrared also allow identification of chemicals and materials. More specific identification is possible if narrow spectral bands are used, or if specific collections of spectral bands are used that are well chosen to discriminate between different chemicals or materials. Such discrimination can be very useful in identifying targets, tracking biological and chemical agents, finding materials, locating plants and in tracking various other materials and objects in the environment.
Hyperspectral imaging attempts to collect image data at a very large number of colors or in many different spectral bands. This technique generates very large amounts of data that is difficult to process. An alternative approach is to make a system that collects a few specific wavelengths using fixed filters in front of detector elements. Such a system can detect specific kinds of objects with specific emission, reflection or absorption spectra. However, this type of system is typically only usable for the specific objects it is designed to detect.
Another approach uses a kind of adaptive spectrometer, as discussed by Deverse at al., xe2x80x9cSpectrometry and Imaging Using a Digital Micromirror Arrayxe2x80x9d, American Laboratory, Vol. 30, No. 21, 1998, pp. S112. This approach allows the optimum spectral response to be chosen to discriminate objects. This system is based on the use of micromirror arrays and can provide choice of spectral sensitivity at least for a linear array of light spots or picture elements. Unfortunately, it is more difficult to use this technique for controlled spectral sensitivity in a two-dimensional image.
What is required is adaptive spectral sensing which is easy to implement for two-dimensional arrays of picture elements. It would also an advance to provide for additional adaptive sensing capabilities, such as adaptive spatial sensing and adaptive temporal sensing.
Accordingly, it is a primary object of the present invention to provide for efficient adaptive spectral sensing in two-dimensional arrays of picture elements.
It is another object to provide for efficient adaptive spatial and temporal sensing in two-dimensional arrays of picture elements.
These and other objects and advantages will become apparent upon reading the ensuing description.
The objects and advantages set forth are achieved by a method for adaptive spectral sensing developed for a two-dimensional image made up of picture elements. The method calls for illuminating at least one of the picture elements with an input light, e.g., light from an object to be examined, and deriving a time-varying spectral signal from the input light for that picture element. Next, the time-varying spectral signal is processed with a time-varying reference signal to obtain a processed output value for the picture element. The output value is then employed for determining a spectral characteristic of the input light. The spectral and reference signals are processed by applying to them a mathematical operation such as convolution, multiplication, averaging, integrating, forming an inner product, matched filtering, addition, subtraction and division.
The time-varying spectral signal is conveniently derived by optical filtering of the input light yielding an optical time-varying spectral signal. The optical filtering can be performed by an optical filter positioned in front of the picture element. In one embodiment, the optical filter is a scanning optical filter and the optical filtering function is performed by scanning. In another embodiment, the optical filter is a Fourier transform scanning optical filter and the optical filtering function involves performing a Fourier transform. The optical filtering can also be combined with other operations, e.g., optically splitting the input light.
Alternatively, the time-varying spectral signal is derived from electrically controlling a spectral detector element.
The spectral detector element can be any suitable photodetector such as a quantum well infrared photodetector, a silicon photodetector or an analog electronic multiplier. In the process of electrically controlling the photodetector the derived time-varying spectral signal can be an electrical time-varying spectral signal such as a voltage, a current, an inductance, a power, an electric field, a magnetic field, a resistance, a capacitance or an inductance. The time-varying reference signal with which the electrical time-varying spectral signal is processed is also in the electrical form; i.e., it is an electrical time-varying reference signal.
In a preferred embodiment, the two-dimensional image of picture elements is formed of an array of pixels. In other words, the picture elements of the image correspond to a number of pixels. It is also convenient that in this embodiment time-varying spectral signals be derived for each of the pixels. The time-varying reference signals used in this derivation can be different for different pixels.
The technique for adaptive spectral sensing can be implemented independently or together with at least one other adaptive sensing technique such as adaptive spatial sensing and adaptive temporal sensing.
The specific embodiments of the invention are described in the detailed description with reference to the attached drawing figures.